1. Field of the Invention
The present invention relates to a planar SQUID (Superconducting QUantum Interference Device) of an oxide superconductor, and more specifically to a sensitive planar SQUID of an oxide superconductor.
2. Description of Related Art
A SQUID is a sensitive magnetism sensor utilizing a Josephson junction, which is capable of measuring extremely weak flux less than flux quantum .phi..sub.0 (=h/2e,h: Planck's constant, e: charge of an electron) by converting the flux into voltage. The SQUID has a superconducting loop and one or two Josephson junctions which are inserted to the superconducting loop. The SQUID which has one Josephson junction is called a RF-SQUID and the one which has two Josephson junctions is called a DC-SQUID.
When the SQUID is used as a magnetic sensor, the SQUID is usually associated with a superconducting flux transformer in order to increase its sensitivity. The superconducting flux transformer is constituted of two superconducting coils of different sizes connected in parallel, which are called a pickup coil and an input coil. The input coil is the smaller one which has a size close to that of the SQUID and is placed on the SQUID. The pickup coil is the larger one which detects flux and is arranged apart from the SQUID. The SQUID and the input coil are usually arranged in a magnetic shield.
The flux detected by the pickup coil is concentrated to the input coil and the SQUID measures the concentrated flux. By this, the substantial sensitivity of the SQUID increases in relation to the self conductance ratio of the pickup coil to the input coil.
FIG. 1 shows a conventional planar DC-SQUID associated with an input coil of the superconducting flux transformer, which is disclosed by M. B. Kechen et al. in Appl. Phys. Lett., 44, 736 (1982). In FIG. 1, the SQUID 1 is mainly constituted of a washer 10 of a square superconducting thin film of which a side has a length of l, which has a square hole 11 at the center, of which a side has a length of .sigma..
The superconducting thin film of the washer 10 overlaps at a portion 19 and the overlap portion 19 is connected to the overlapped portion of the washer 10 through two Josephson junctions (not shown) connected in series. The washer 10 has two tongue portions 14 and 15, in order to connect the SQUID 1 to a signal processor (not shown).
An input coil 3 of a superconducting flux transformer is arranged on the SQUID 1 and isolated from the SQUID 1. The input coil 3 is formed of a helical rectangular shaped superconducting coil. The input coil 3 is connected to the pickup coil (not shown) of the superconducting flux transformer.
In the above mentioned conventional planar SQUID, the length l of the side of the washer ranges from 100 to 500 .mu.m and the length .sigma. of the side of the hole ranges from 10 to 100 .mu.m and the ratio of l to .sigma. ranges 10 to a few tens.
The pickup coil of the superconducting flux transformer has a size on the order of 1 mm. Therefore, it is difficult to miniaturize the SQUID associated with the superconducting flux transformer or to form it into one tip device. In addition, a fine and complicated processing is needed to realize the SQUID on which the input coil of the superconducting flux transformer is integrally formed.
However, the conventional SQUID does not has sufficient sensitivity alone that it can detect and measure the biological magnetism without the superconducting flux transformer.